Tuesday, December 19, 2006

A Circulation Response Looms

Global tropical SSTs generally remain above average, with the greatest anomalies across the Indo-Pacific region especially from the central equatorial Pacific to the west coast of South America. Magnitudes across the latter are ~plus 1-3C and extend to 125m deep (up to 5C) per latest TAO buoy data as part of the warm ENSO signal. Interestingly, deep cool anomalies ~1-2C are appearing west of the date line and the 20C isotherm depth has shoaled to about 150W. The warm-cool-warm SST distribution observed for the past several months has become less distinct suggesting our warm event may have already peaked. In any case, actual SSTs 29C (threshold for supporting sustained tropical convection) and higher extend from the South Pacific into eastern Indonesia and over portions of the Indian Ocean.

Tropical convective forcing remains very complicated. Per full disk satellite imagery and monitoring tools (such as coherent modes and velocity potential Hovmollers) a relatively fast dynamical signal (~15-20m/s) that was moving east through the Western Hemisphere has emerged into Africa and the Indian Ocean. An intense region of deep convection (3-day OLRA less than minus 70W/m**2) has resulted across the equatorial Indian Ocean centered ~70E (including TC Bondo across the South Indian Ocean) and is projecting onto a MJO per Wheeler phase plot. SST boundary forcing and Southern Hemisphere frontal activity has allowed another burst of convection to persist ~0/160E while diurnal activity still continues across the Amazons of northern South America.

To sum up, we continue to see two primary regions of active forcing, Indian Ocean and west of the date line (suppression in between), being modulated by at least MJO-like variations, Kelvin waves and flare-ups with time scales varying anywhere from 5-35 days. The pattern of the 2 regions of active forcing has loosely been drifting east since October (~ 30deg), particularly the Indian Ocean enhancement-Indonesian suppression portion. The Indian Ocean has tended to dominate the central Pacific forcing, and all of these kinds of behaviors are not consistent with the composite tropical convective response to a developing warm ENSO.

A response to the multiple regions of tropical forcing has been for zonal mean easterly wind anomalies to increase throughout much of the equatorial atmosphere since roughly the start of this month (magnitudes approaching 10m/s at 200mb starting about December 13th). Anomalous twin upper tropospheric anticyclones located around 60E, 180 and 60W (per 150mb 12/19/06 daily mean vector wind anomalies) are supporting these easterlies and are also part of the tropical-extratropical circulation response. These easterlies are beginning to propagate poleward off the equator while zonal mean westerly wind anomalies remain across the subtropical and midlatitude atmospheres. Based on the ESRL/PSD reanalysis data AAM plots, as of December 16th the global tendency had reached about minus 15 Hadleys with contributions not only from the equatorial zonal mean easterlies but also weakly from the global mountain and frictional torques (see plots for details). I think GSDM Stage 4 best describes the current weather-climate situation.

My thoughts remain that sometime during weeks 2-3, going along with the seasonal cycle, a consolidation of the tropical forcing may occur centered ~150-160E as it shifts south. Heading into the first week of January we may see a large region of strong tropical thunderstorm activity extend from the South Pacific to just east of Indonesia. This may maintain an increased risk for tropical cyclones across the South Indian Ocean as well as the South Pacific to the east coast of Australia. Afterwards, this whole region may shift southeast along the SPCZ while the ITCZ and much of Brasil are active. I would also expect to the Indian Ocean and Indonesia to remain periodically active. In fact, at some point, say February-March, much of the East Indian Ocean and Indonesia may become quite active while the central Pacific starts to shut down. What is left of the dynamical warm ENSO signal may be centered near the Americas.

I think we are transitioning to GSDM Stage 1 as I type. Uncertainty for any predictive insight remains extremely high. However, after Christmas heading into the first couple of weeks in January 2007, I would almost be surprised if a GSDM Stage 2 situation did not evolve. That would suggest ridge amplification just off the North American west coast into Alaska with a few digging synoptic troughs across the Rockies and Plains. Ramifications would include true Arctic air penetrating into the lower 48 states with lots of wintery precipitation to go along with it, especially for the eastern 2/3rds of the country. The Pacific Northwest may still see bouts of precipitation as the troughs come inland. Afterwards the DREADED extended combined jet from Asia to the west coast of North America (with split flow across the continent) may appear which is typical of GSDM Stage 3 that is most probable during a mature global response to a warm ENSO. At that point the precipitation emphasis would be along the USA west coast as well as the Deep South. While the latter may verify for a January-March seasonal mean, I could see the February-March period going back to GSDM Stages 4-1.

Appendix

The following is a link to our recently accepted paper by MWR which discusses the GSDM (Weickmann and Berry 2006, in press).

In the beginning of Section 2 of that report we give a summary description of the Global-Synoptic-Dynamic Model (GSDM) of subseasonal variability. I hope to get an update out by late next week. As stated previously, while I am at ESRL/PSD we hope to post at least short writings on this Blog every other day in support of the Hydrometeorological Testbed Project (HMT -- please see

1 comment:

"The Indian Ocean has tended to dominate the central Pacific forcing, and all of these kinds of behaviors are not consistent with the composite tropical convective response to a developing warm ENSO".

Tho, I have a question to ask. What would be the normal composite tropical convective response to a developing warm ENSO? How different is the MJO this time compare to that of previous ENSO events during the boreal winter?